Conventionally, as disclosed in U.S. Pat. No. 5,492,097, an electronically controlled throttle apparatus is provided with an opener-side mechanism (rimp-home mechanism) which opens the throttle valve. With such a mechanism, if current supply to a driving motor is interrupted for some reason, a throttle valve is mechanically brought into a predetermined position (intermediate stop position). A plurality of springs of different biasing forces may be used in such a mechanism. This predetermined position is an intermediate position between a fully closed position and a fully open position of the throttle valve. Thus, the internal combustion engine is prevented from being immediately stopped and the vehicle can be driven to a turnout (e.g., repair shop).
Such a mechanism requires two lever members (opener member and intermediate stop member) and two spring members (spring for opening function and spring for returning function). This poses increases in the number of parts and cost. Further, the intermediate stop member to be abutted against a locking portion on the throttle housing side adopts complicated construction. That is, the stopper member is so constructed that the intermediate stop position in the throttle valve will be set through a part of abutting against the opener member. This leads to a problem that the opening angle of the throttle valve in the intermediate stop position is varied by variation in component parts or the like as well.
It is proposed to reduce the number of parts in the opener mechanism of an electronically controlled throttle apparatus for the simplification of the construction thereof. Further, it is proposed to enhance the accuracy of the opening angle of a throttle valve in the intermediate stop position. These proposals adopt a coil spring structure are the subject of U.S. 2002/0078923 A1 (EP 1 219 803 A2, JP-P2002-256894A). In this structure, a joint between a first spring portion having a returning function and a second spring portion having a opening function is bent substantially in the reverse U-shape. Thus, a U-shaped hook portion which is fixed in an intermediate stop position defined in a throttle housing is formed. The ends of the coil spring (one end of the first spring portion and the other end of the second spring portion) are wound in different directions.
However, when the throttle valve is closed from the intermediate stop position to the fully closed position, a problem arises. The U-shaped hook portion of the coil spring is secured on one end of a housing hook, and the opener member is rotated together with a spring gear-side hook which constitutes one end of the second spring portion. Thereby, biasing force is produced in such a direction that the throttle valve is returned from the fully closed position to the intermediate stop position. At this time, a spring inside circumferential guide which retains the inside diameter portion of the coil spring is greatly moved relative to the inside circumferential surface of the second spring portion of the coil spring.
Simultaneously, relative motion is produced between an engaging portion which is integrally formed on the valve gear and engages with the U-shaped hook portion and the U-shaped hook portion of the coil spring. Relative motion is also produced between lateral displacement prevention guides for arresting the displacement of the U-shaped hook portion in the axial direction (lateral direction) and the U-shaped hook portion. This increases sliding resistance. Therefore, when the throttle valve is closed from the intermediate stop position to the fully closed position, relative motion is produced between the outside circumferential surface of the spring inside circumferential guide and the inside circumferential surface of the second spring portion of the coil spring. Further, relative motion is also produced between the engaging portion and the lateral displacement prevention guides and the U-shaped hook portion of the coil spring. As a result, great sliding resistance is produced, which causes throttle valve inoperativeness.